This invention relates to a process for separating propylene from mixtures of low molecular weight hydrocarbons.
The separation of propylene from low molecular weight hydrocarbon mixtures is an extremely important and large volume operation in the chemical and petrochemical industries. Catalytic cracking and steam cracking are among the most common and large scale processes leading to these mixed hydrocarbon streams. The need to recover propylene from propane-containing streams, in particular, is one of high economic significance in the synthesis of polypropylene elastomers. However, despite the close proximity in boiling points between propylene and propane, these components are presently separated by fractional cryogenic distillation. The large size of the columns and the energy intensity of this distillation process have, however, created large incentives for alternative means of effecting these separations in a more efficient and cost-effective manner.
Some of the leading alternatives to fractional cryogenic distillation involve the use of adsorbents that exploit their ability to adsorb some of the components selectively. This has given rise to various forms of pressure or temperature swing adsorption (PSA/TSA) processes in which the mixture is first passed through an adsorbent material under conditions where one or more of the components are selectively removed. The loaded material is then typically exposed to a lower pressure and/or higher temperature environment where the adsorbed components are released and recovered at a higher purity level. Economic viability requires adsorbent materials that can deliver high selectivity, high adsorption capacity, and short duration cycles. An additional and critically important requirement is that the material should not catalyze chemical reactions that might lower the recovery of the desired components and/or render the adsorbent inactive.
Among the adsorbents which have been proposed for the recovery of propylene from hydrocarbon mixtures are ion exchange resins, mesoporous solids, activated carbons, and zeolites. Ion exchange resins and mesoporous solids usually exploit equilibrium adsorption properties in which one of the components is selectively adsorbed over suitably dispersed chemical agents. They principally rely on the adsorption affinity of cationic active centers such as Ag and Cu ions for the double bond in propylene (xcfx80-complexation). The characteristic time associated with the adsorption cycle is that required to bring the mixture close to thermodynamic equilibrium with the adsorbent. The relative rates of diffusion of the various components within the adsorbent are of secondary importance. Activated carbons and zeolites, on the other hand, typically resort to a combination of adsorption affinity and diffusion control. Two related cases of diffusion control are of interest here. In one extreme case, the separation is achieved by excluding the diffusion of some of the components into the adsorbent. The second case exploits a sufficiently large difference in diffusion rates to allow the preferential uptake of some of the components within a predetermined adsorption time. Thus, carbons are usually activated to very high surface area forms in order to provide textural properties and pore sizes that maximize adsorption while selectively controlling diffusion. Aluminosilicate and silicate zeolites have become even more attractive than activated carbons because of the ever increasing possibilities afforded by new synthetic routes, which allow for a more flexible and precise control of chemical composition, pore size, and pore volume. The tetrahedrally coordinated atoms in these microporous materials form ring structures of precise dimensions that selectively control the diffusional access to the internal pore volume.
Eight-membered ring zeolites, in particular, have been actively investigated for the separation of small molecular weight hydrocarbons because their window sizes are very comparable to molecular dimensions and because they can provide high adsorption capacities. A typical example is the Linde type A zeolite which is characterized by a set of three-dimensional interconnected channels having 8-membered ring window apertures. The effective size of the windows depends on the type of charge-balancing cations. This has given rise to the potassium (3A), sodium (4A), and calcium (5A) forms, which have nominal window sizes of about 3 xc3x85, 3.8 xc3x85, and 4.3 xc3x85, respectively. Thus, for example, EP-B-572239 discloses a PSA process for separating an alkene, such as propylene, from a mixture comprising said alkene and one or more alkanes by passing the mixture through at least one bed of zeolite 4A at a temperature above 323xc2x0 K to preferentially adsorb said alkene and then desorbing the alkene from the bed. EP-A-943595 describes a similar process in which the zeolite adsorbent is zeolite A having, as its exchangeable cations, about 50 to about 85% sodium ions, about 15 to about 40% potassium ions and 0 to 10% of other ions selected from Group IA ions (other than sodium and potassium), Group IB ions, Group IIA ions, Group IIIA ions, Group IIIB ions and lanthanide ions.
In zeolites, it is well-accepted that the control of window size is critically important for achieving high separation selectivities. For a given zeolite structure type, the effective size of the windows can be tuned by partially blocking or unblocking the windows with pre-selected charge-balancing cations. This provides a reasonable but not necessarily optimal control of window size because of the inherent difficulties of precisely placing these cations in a uniform manner throughout the structure. More importantly, the propensity of these cations to promote or participate in undesired reactions that may impart catalytic activity to the material can lead to detrimental oligomerization and polymerization reactions of olefins. These reactions not only lower the recovery of the desired components, they are also likely to render the adsorbent inactive. The double bonds in the olefins are particularly prone to attack even by mildly acidic sites and this may severely limit the temperature and partial pressures at which the separation process can be carried out.
In an effort to control chemical reactivity more reliably, there is a growing interest in the use of non-acidic, all-silica zeolites. Since these siliceous zeolites require no extra-framework balancing cations, the size of the windows is much more uniform throughout the crystals and largely determined by the crystal structure. Thus, for example, the potential of deca-dodecasil 3R (xe2x80x9cDD3Rxe2x80x9d) for separating propane and propylene has been very recently reported. See Zhu, W., Kapteijn, F., and Moulijn, J. A. xe2x80x9cShape Selectivity in the Adsorption of Propane/Propene on the All-Silica DD3Rxe2x80x9d, Chem. Commun. 2453-2454 (1999). This crystalline microporous silicate has a two-dimensional pore system formed by 8-membered rings of tetrahedrally coordinated atoms with a nominal window size of 3.6 xc3x85xc3x974.4 xc3x85 (see Atlas of Zeolites Framework Types, Fifth Revised Edition, pages 108-109, 2001). Diffusion and adsorption measurements on this material indicate that only propylene is able to access the interior of the crystallites. The exclusion of propane was suggested as the basis for a very selective separation scheme. The size of the deca-dodecasil 3R windows, however, appears to be so close to the effective kinetic diameter of propylene that the diffusion rates are very low and this could lead to undesirably long adsorption and desorption cycles. Cycle duration can, in principle, be reduced by appropriate reductions in crystal size but such changes are not always possible with the known synthetic procedures.
The advantages of reactivity control and size exclusion afforded by materials like DD3R may not be sufficient to develop an effective separation process. The window size also has to be optimally controlled such that short duration cycles are achieved. Work by the present inventors has shown that a more optimal control of window size, with a simultaneous control of chemical reactivity, can be obtained with certain crystalline microporous materials containing phosphorous in the framework. For example, aluminophosphate AlPO-34, which is isostructural with chabazite (CHA), has pores defined by a three-dimensional interconnected channel system of 8-membered rings. Since the numbers of Al and P atoms in the unit cell of AlPO-34 are the same, there is no need for balancing cations. The lack of Brxc3x6nsted acidity in this material not only permits its use as an adsorbent at higher temperatures, it also more properly tailors the size of the windows by changes in the bond angles and bond lengths of the tetrahedrally-coordinated atoms and the bridging oxygens. For example, compared to a pure silica CHA, whose pore size of 3.50 xc3x85xc3x974.17 xc3x85 may be too small for rapid transport of propylene, AlPO-34 exhibits a slightly larger pore size of 3.86 xc3x85xc3x974.18 xc3x85. (These window size dimensions were obtained by the Distance-Least-Square (DLS) method, constraining the cell size to that measured for the material). It has now been found that this seemingly small increase in window size is critical to enhancing propylene diffusivity without appreciably enhancing propane diffusivity.
The window sizes in these phosphorous-containing materials can be further modified by suitable atomic substitutions that change bond lengths and bond angles while preserving the crystalline structure. Thus, for example, the complete replacement of Al by Ga in the synthesis mixture to give GaPO-34, which is isostructural with AlPO-34, leads to another very effective material for separating propylene from propane. Some of the advantages of AlPO-34 and GaPO-34 can also be found in AlPO-18 (AEI), which has a structure closely related to that of CHA and also comprises a three-dimensional interconnected channel system of 8-membered rings having DLS apertures of 3.61 xc3x85xc3x974.47 xc3x85. Once again, unlike the situation in aluminosilicates, but similar to AlPO-34 and GaPO-34, these dimensions in AlPO-18 represent the actual size of the windows because there is no need for balancing cations.
The diffusivity of a porous crystalline material for a particular sorbate is conveniently measured in terms of its diffusion time constant, D/r2, wherein D is the Fickian diffusion coefficient (cm2/sec) and r is the radius of the crystallites (cm) characterizing the diffusion distance. In situations where the crystals are not of uniform size and geometry, r represents a mean radius representative of their corresponding distributions. The required diffusion time constants can be derived from standard sorption kinetics measurements as described, for example, by J. Crank in xe2x80x9cThe Mathematics of Diffusionxe2x80x9d, 2nd Ed., Oxford University Press, Great Britain, 1975 or by frequency response methods as described, for example, by Reyes et al. in xe2x80x9cFrequency Modulation Methods for Diffusion and Adsorption Measurements in Porous Solidsxe2x80x9d, J. Phys. Chem. B. 101, pages 614-622, 1997.
In accordance with the invention, it has now been found that AlPO-34 and AlPO-18 and their gallium-containing counterparts have the capability of achieving very effective separation of propylene and propane through a unique combination of diffusion time constants in which the time constant for propylene is not only much higher than for propane, but it is at the same time also high enough to permit short adsorption/desorption cycles that are economically viable.
U.S. Pat. Nos. 6,293,999 and 6,296,688 disclose the use of AlPO-14 (AFN) for separating propylene from propane. However, although AlPO-14 possesses a set of three-dimensional interconnecting 8-ring channels, only one of them seems large enough to host propylene and therefore AlPO-14 should exhibit a low propylene adsorption capacity. Moreover, with a nominal window size dimension of only 3.3 xc3x85xc3x974.0 xc3x85 (Atlas of Zeolites Framework Types, Fifth Revised Edition, pages 36-37, 2001), the diffusion of propylene should be slow and associated with undesirably long adsorption cycles.
According to the invention there is provided a process for separating propylene from a mixture comprising propylene and propane comprising the steps of:
(a) passing the mixture through a bed of an adsorbent comprising a porous crystalline material having a diffusion time constant for propylene of at least 0.1 secxe2x88x921, when measured at a temperature of 373xc2x0 K and a propylene partial pressure of 8 kPa, and having a diffusion time constant for propane, when measured at a temperature of 373xc2x0 K and a propane partial pressure of 8 kPa, less than 0.02 of said diffusion time constant for propylene; and then
(b) desorbing the propylene from the bed.
Preferably, the porous crystalline material is non-acidic.
Preferably, the porous crystalline material is selected from the group consisting of aluminophosphates, gallophosphates, galloaluminophosphates, metalloaluminophosphates and metalloaluminophosphosilicates.
Preferably, the porous crystalline material is selected from the group consisting of AlPO-34, GaPO-34, AlPO-18 and GaPO-18.